1. Field of the Invention
This invention relates in general to data storage and processing, and more particularly to a method, apparatus and frame format to prevent data loss in a removable media library system due to incorrect configuration of devices.
2. Description of Related Art
The implementation of new technology in magnetic storage products has meant that the density of data written to magnetic storage media has increased by orders of magnitude in the last ten or fifteen years. The ability to record high density media, e.g., ten gigabytes or more on one physical volume, has led to reducing costs in physical hardware as well as in handling and management resources.
Nevertheless, certain problems still remain with this relatively new technology. For example, under certain conditions, data can become lost due to incorrect configuration of the devices. In a Removable Media Library (RML), hosts request the mounting of volumes on devices. However, if the devices are configured incorrectly, the library will mount the wrong volume serial numbered cartridge on the device. This means that the host will write data on the wrong physical volume. This causes the data to become lost because the next time the host requests the volume, it will request the one it thought it had written data to, not the one that was actually mounted.
For example, a host requests the RML to mount volume 1 on device A, and to mount volume 2 on device B. The RML uses the external label to mount the volumes on the requested devices and the RML successfully mounts the volumes as requested. However, due to an incorrect configuration, the RML thinks device A is device B and visa versa. The host writes the internal label on each volume, but because of the configuration problem, volume 1 now has an internal label of 2 and volume 2 has an internal label of 1. Thus, the data written by the host is put onto the wrong volumes. Subsequently, the volumes are dismounted. Later when the host asks for Volume 1 to be mounted again, the RML mounts the volume with external label 1 but with an internal label of 2. The data intended for Volume 1 is now lost.
A specific example of an RML is a virtual tape server (VTS). In a VTS, the hardware is transparent to the host and the user, and the VTS requires little external management except through the library management element of the tape library into which a VTS is integrated.
In hierarchical storage systems, such as a VTS, intensively used and frequently accessed data is stored in fast but expensive memory. One example of a fast memory is a direct access storage device (DASD). In contrast, less frequently accessed data is stored in less expensive but slower memory. Examples of slower memory are tape drives. The goal of the hierarchy is to obtain moderately priced, high-capacity storage while maintaining high-speed access to the stored information.
In the VTS system, a host data interface, a DASD file buffer, and a number of tape devices are provided. When the host writes a logical volume, or a file, to the VTS, the data is stored as a resident file on the DASD. Although the DASD provides quick access to this data, it will eventually reach full capacity and a backup or secondary storage system will be needed. An IBM 3590 tape cartridge is one example of a tape device that could be used as a backup or secondary storage system.
When the DASD fills to a predetermined threshold, the logical volume data for a selected logical volume, typically the least recently accessed by the host, is removed from the DASD to free space for more logical volumes. The selected DASD file is then appended onto a tape cartridge, or a physical volume, with the original left on the DASD for possible cache hits. When a DASD file has been appended to a tape cartridge and the original remains on the DASD, the file is "premigrated".
When the host reads a logical volume from the VTS, a cache hit occurs if the logical volume currently resides on the DASD. If the logical volume is not on the DASD, the storage manager determines which of the physical tape volumes contains the logical volume. The corresponding physical volume is then mounted on one of the tape devices, and the data for the logical volume is transferred back to the DASD from the tape (recall).
Tape servers may use an engine to move data between the DASD and tape drives in a virtual tape server (VTS) environment. For example, the IBM Virtual Tape Server (VTS) uses the IBM Adstar Distributed Storage Manager (ADSM) as its engine to move data between the DASD and IBM 3590 tape drives on the VTS. In such a system, the VTS uses the storage manager client on the DASD, e.g., the ADSM Hierarchical Storage Manager (HSM) client, and a distributed storage manager server attached to the tape drives to provide this function.
Within a Virtual Tape Server (VTS) like system, the consequences of a mis-configured set of devices is compounded since a single physical volume contains multiple logical volumes (100+). Each physical volume whose internal label does not match the external label results in multiple logical volumes becoming lost and a great impact to a customer. In a VTS, it is the responsibility of the VTS to ensure that the devices are configured correctly since it is the host system for the physical devices.
It can be seen then that there is a need for a method to determine if the devices are mis-configured and to prevent the use of the mis-configured devices.